net: phy: restore mdio regs in the iproc mdio driver

The mii management register in iproc mdio block
does not have a retention register so it is lost on suspend.
Save and restore value of register while resuming from suspend.

Fixes: bb1a6197

 ("net: phy: Initialize mdio clock at probe function")
Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>